I fail to see how the difficulty is "arbitrary" by any means.
Arbitrary difficulty is any of the following (For MMOs):
Pure gear/math checks (IE: Can't kill a boss because it has more HP regen (Or an enrage timer) than you put out in optimized DPS, or tank checks).
Ping difficulty (The lag is real)
Overpopulated mob spawns
Imbalanced repair costs/money gain/etc.
Anti-grouping mechanics (Forced solo instancing, bad loot mechanics, bad boss-to-player stat scaling)
Difficult skill rotations.
In-depth class and game mechanic knowledge. (Your complaint).
If you don't enjoy a challenge, don't play the game - but don't pretend it's some failing of the combat system, which is where this game clearly shines. If you want to blame BnS for anything, blame it for the overall bland feeling outside of combat, horrible loot system, and massively imbalanced sell/buy prices at vendors during early levels. The combat system is by far better than most MMOs.